Micro-Electro-Mechanical Systems or short MEMS, is a engineering that is basically a combination of electro-mechanical stuffs and mechanical technology at a micro graduated table. The engineering used for fiction is derived from the IC fiction procedures and typically ranges from one micrometer to several millimetres. The MEMS engineering finds its usage in assorted applications where for case, micro detectors and/or micro actuators are needed. As its name suggests, MEMS devices are basically mechanical devices that can dwell of fixed parts or a combination of fixed and traveling parts. However this besides gives rise to dependability issues due to both electrical as mechanical issues [ 2 ] s.a. stuff weariness, stiction, dazes etc. The dependability can be greatly enhanced [ 1 ] turn outing the engineering can be suited for usage in high terminal applications. The end of this essay is to present and familiarise the reader to an extent with the operation and the dependability issues of a MEMS accelerometer. First the working of a MEMS accelerometer will be covered after which its dependability will be considered. Here assorted dependability issues will be considered after which……..some…………….solutions will be given on how to cover with…..some…..of…..the dependability issues. Finally a decision will be drawn if the presented solution…….solutions………..are bettering the dependability.
MEMS engineering devices have established a repute over the old ages as capable detector and actuator devices doing assorted information garnering techniques possible. The engineering enables applications to garner information from the environment in assorted spheres merely to change over them to the electrical sphere. Following to the electrical sphere, the detectors can be fabricated to observe mechanical, thermic, chemical, magnetic and optical signals.
Give some more illustrations about the usage and demo some images etc. particularly mention the usage in automotive industry
One noteworthy illustration that possibly talk easy to the head is the usage of MEMS detectors in the modern smartphones. The detector enables the phone to observe up from down for assorted maps s.a. exchanging to a larger screen, GPS applications and game characteristics where the phone detects the tilting for playing some games.
Figure… ..shows the usage of accelerometers germinating over the old ages.
[ 4 ]
As the figure points out, the MEMS accelerometer had made its first visual aspect as a clang detector in an airbag. From there it evolved to be used even in playthings where it needs to be inexpensive and is produced in a high volume.
However an application in toys doesn’t need to be extremely dependable unlike the usage in application for automotive terminals s.a. the airbag where a life can be saved depending on the right operation of the detector.
1. MEMS accelerometer
Figures…shows a basic MEMS capacitive accelerometer.
[ 4 ] ………… .
The MEMS detector shown uses two home bases connected to the cardinal mass w.r.t the fixed home bases as differential capacitances. The electrical capacity is given by
The two home bases attached to the cardinal mass signifier series electrical capacities with the fixed home bases. When there is acceleration the mass will travel in the positive or negative way depending on the motion. This in bend will ensue in positive or negative electrical capacity and depending on the distance the capacitances are smaller or larger. Thus for positive motion one capacitance will increase positively while the other will increase negatively, or frailty versa for negative motion. When there is no acceleration the movable mass will be back at its place due to the snap belongingss of Si which consequences in a place equidistance between each outer plates. This will ensue in zero electromotive force on the cardinal home base as the electrical capacity on each side are equal ( except for the sine ) . This is because the outer home bases can be driven with a square moving ridge with the coveted frequence so that one home base sees a positive signal while the other sees a negative signal, and frailty versa. For positive motion the end product square moving ridge is in stage with the input signal resulting in a greater magnitude when demodulated. For negative motion this is the antonym.
The described device has merely two outer home bases which means that the sensitiveness will be low. To increase the sensitiveness the figure off traveling fingers ( outer plates ) must be increased [ 1 ] .
However every bit will be shown in the text, the more moving parts in the accelerometer the higher the chance of failure.
2. Common MEMS failure mechanisms
Although the MEMS engineering has proved its worth, there are many failure mechanisms that need to be considered when planing to hold a qualitative merchandise at the terminal of the processing. Table… [ 6 ] summarizes the common failure mechanisms of MEMS devices:
Overload, daze ; Corrosion ; Fatigue
Van der Waals force ; Capillary force ; Chemical bonding ; Electrostatic charging ; Residual emphasis
Electric emphasis ; Radiation ; Improper handling
Adhesion ; Abrasion ; Corrosion ; Surface weariness
Creep and weariness
Intrinsic emphasis ; Applied emphasis ; Thermal emphasis
Electric short and unfastened
Dielectric stuff debasement ; ESD, high electric field ; Electromigration ; Oxidation
Intrinsic ( e.g. , crystal growing ) ; Manufacturing-induced ; Usage environment-induced
As the accelerometer is considered as the instance survey, merely some of the failure mechanisms are of involvement. The most of import failure mechanisms are Mechanical breaks. Following to that stiction is besides of import as the accelerometer contains traveling parts. In ulterior subdivisions some solutions will be presented on how to better some of the failure mechanisms.
Mechanical break is the most common failure manner and largely is a consequence of Shocks. Dazes are fundamentally sudden accelerations. While usually an accelerometer operates under conditions merely holding to cover low g-shocks s.a. in consumer electronics, auto clang detectors need to defy much more than that to be dependable. Apart from dazes the mechanical failure manner can besides be a consequence of in-use-stiction as a consequence of suspended parts doing contact that under normal conditions doesn’t occur. The latter will be shows in the following subdivision.
Reducing mechanical breaks
To better the MEMS accelerometer assorted methods can be used. One normally used method is to restrict the motion of the cogent evidence mass by implementing a stopper. Figure…..shows an execution of the latter.
[ 7 ]
The T-shaped construction is the stopper. By implementing the stopper, the break of the back uping beams can be avoided as the maximal emphasis on the beams given by the snap is non exceeded. Following to restricting the emphasis it besides prevents the in-use-stiction as the moving parts are prevented from doing contact with the fixed parts.
Stiction is one of the most common failure mechanisms in MEMS and occurs where suspended constructions are used ( traveling parts ) . When different parts s.a. the fixed fingers in the accelerometer make contact, these can stay affiliated to one another. Figure…… . [ 6 ] …shows some illustrations of sidelong or/and perpendicular stiction.
Fig… Examples of assorted stiction ( a ) no stiction ; ( B ) merely sidelong stiction ; and ( degree Celsius ) lateral and perpendicular stiction [ 6 ] .
The stiction occurs peculiarly in micromachined constructions where the surface country to volume ratio is big ensuing in a typically little stiffness of reconstructing springs. The stiction can hold assorted causes and most of these can be due to processing, elevated RH degrees and dazes. The adhesion occurs either due to van der Waals, electrostatic forces ( trapped charge ) , capillary forces or a combination of these [ 7 ] .
As the failure mechanisms of MEMS devices are progressively researched, many ways have been proposed to cut down stiction. Some of these where:
Modifying the structural stiffness
Design of the traveling parts with higher stiffness to get the better of the stiction force
Surface chemical science
Using hydrophobic surfaces to forestall H2O from coming into contact with the MEMS structures as H2O causes capillary force and hence stiction [ 6 ] . Alternatively a hermetic waterproofing can be used
Rough surfaces reduces the Van der Waals force and with that the stiction
Implementing a stopper to restrict the motion of the cogent evidence mass ( see old subdivision )
Anti stiction coating
Definition of the dependability
Before discoursing the dependability of an accelerometer, the definition of dependability must be considered. Reliability is defined as the chance of a certain merchandise to execute harmonizing to its specifications within typical operating conditions for the expected life-time [ 2 ] .
The expression for this is given by
As for the failure rate, which is defined as the ratio of the entire figure of failures to the entire operating clip [ 1 ] . Thus ?
For measuring the dependability there are several standard theoretical accounts that can be used, viz.
The exponential dependability distribution
The binomial dependability distribution
The Poisson dependability distribution
Weibull dependability distribution
In this text the exponential signifier is used for the dependability analysis as its less complex than other lifetime distribution theoretical accounts. Then the dependability is given by
In world the failure rate is besides a map of clip. The failure rate ( as a map of clip ) follows the behaviour of a bathing tub curve [ 1 ] .
[ 2 ]
The infant mortality part is caused by merchandise defects and perchance besides by cognize how restrictions like when proving paradigms [ 2 ] , as for the wear-out part, this is merely as its name suggests.
Here the chance of no failure before clip T is given by [ 9 ]
When ciphering the failure rate for increasing smaller intervals of clip, the jeopardy rate H ( T ) is obtained, with
However, because we are sing the exponential failure distribution, the jeopardy rate will be changeless and equal to the failure rate. Here we can see merely the utile clip from the bathing tub curve, that is, most infant mortality failures ( with ruinous defects ) can be filtered out during fabricating trials [ 1 ] . This means that we can see the failure rate to be more changeless and therefore the instantaneous jeopardy rate H ( T ) is merely the failure rate.
4. Dependability of a capacitive accelerometer
As was stated in the Introduction subdivision, the capacitive accelerometer will be used for the chance analysis since it’s widely used and needs to be dependable adequate particularly in life endangering state of affairss s.a. auto clangs. The structural diagram of an accelerometer with 2N-fingers is shown in Figure… .
[ 8 ]
Here the accelerometer consists of fixed and movable parts which all demand to be fault-free. The accelerometer consists of 4 ground tackles, 4 back uping beams, 2N-fixed fingers, one movable cardinal mass and N-movable fingers attached to the cardinal mass. As stated before, utilizing more fingers increases the sensitiveness of the accelerometer.
Here the system can be considered as a series theoretical account [ 1 ] because if any of the constituents fail, the whole system fails.
For simpleness, the fixed parts can be considered holding perfect dependability as most defects are filtered out during the fabricating trial while the traveling parts can hold a higher dependability of failure. The dependability is so given by ( Rdegree Fahrenheitcorresponds to the traveling fingers )
Therefore the entire failure rate is merely the amount of the failure rates which is given by
As can be noted, there is a trade-off between the sensitiveness and the failure rate as both are dependent on n-fingers. As the figure of fingers can be increased to better the sensitiveness, this will convey the dependability down.
5. General solution
As discussed antecedently the mechanical break and stiction are thermoset of import failure mechanisms. Here it was shows how to cut down these failures and hence besides the dependability.
However if the device fails anyway so it’s considered useless. Unfortunately this can hold serious effects if it’s used in airbag systems and therefore the dependability still needs to be higher. To make this a more general attack can be used. This attack makes usage of ‘spare parts’ . If the accelerometer is broken up into smaller faculties connected together with some excess faculties, the latter can be used to replace the faulty faculties. As the faculties can be connected to some circuitry that would feel if the moving parts are faulty, a faulty faculty can be excluded and replaced by a excess one and this procedure is repeated until the redundant parts are used for replacing. This method is known as BISR ( built in self fix ) . Figure…..illustrates this.
Here the k-out-of-n redundancy theoretical account can be used for the analysis [ 1 ] . The k-out-of-n system consists out of n-modules, withK-modules non-faulty to guarantee the right operation of the MEMS system.
When utilizing such a system, until all the excess faculties are used and at least one faculty from the chief device is faulty at the clip, so the system can no longer map decently. This means that the system, unlike when merely one chief faculty is used, is a parallel system. Figure….illustrates this.
As the figure shows a 3-out-4 system is assumed. Notice that merely the moving parts are considered. That’s similar to the one-module device we can concentrate on the moving parts merely as these cut down the dependability at higher rate than the fixed parts.
Again the dependability of one faculty can be done similar as for the one faculty device antecedently discussed in text. The dependability is so given by ( Rdegree Fahrenheitcorresponds to the traveling fingers )
Therefore the entire failure rate is merely the amount of the failure rates which is given by
Notice that unlike earlier, the failure rate is reduced as the figure of traveling fingers are increased for better sensitiveness. This means that the trade-off that had to be made earlier is non made any more and a much better dependability is obtained.
The entire dependability of a system is given [ 1 ] by
Where m is the excess faculty. Therefore in this instance the dependability is given by
Inserting the exponential signifier of the dependability for a faculty
However, utilizing this method besides increases the usage of the figure of back uping beams which in bend reduces the dependability. But in the visible radiation of the improved dependability by utilizing the BISR attack, the overall dependability increases more than is decreases [ 1 ] doing this method really utile.
MEMS have proven to be rather utile. However like any merchandise dependability issues can originate. The dependability of MEMS in toys doesn’t need to be all that high while for applications in automotive this needs to be really dependable as a unrecorded can depend on it when used in airbag systems. It has been shown that first it’s best to see the failure mechanisms and to better these which makes a longer lifetime possible. To better the dependability even further the BISR attack can be considered.
From this it shows that the dependability can be really high and in the hereafter this attack might be considered to better other types of MEMS detectors.
[ 1 ] X. Xiong, Y. Wu, and W. Jone, “Reliability analysis of self-repairable MEMS accelerometer, ” in Proceedings of the 21st IEEE International Symposium on Defect and Fault Tolerance in VLSI Systems ( DFT ’06 ) , pp. 236–244, October 2006
[ 2 ] Jacopo Iannacci, “Reliability of MEMS: A position on failure mechanisms, betterment solutions and best patterns at development level” , Center for Materials and Microsystems – CMM, Fondazione Bruno Kessler – FBK, Via Sommarive, 18, 38123 Povo, Trento, Italy, accepted 2014
[ 3 ] Frank CHoLLET, Haobing LIU“A ( non so ) short direction to MEMS” , Creative Commons, version 5.1, 2013
[ 4 ] Michael Kraft, Neil M. White “MEMS for automotive and aerospace applications” , Woodhead Publishing Limited, Cambridge, 2013
[ 5 ] X. Xiong, Y. Wu, and W. Jone, “Material weariness and dependability of MEMS accelerometers, ” in Proceedings of the 23rd IEEE International Symposium on Defect and Fault Tolerance in VLSI Systems ( DFT ’08 ) , pp. 314–322, October 2008
[ 6 ] Ynhan Huang et Al. “MEMS Reliability Review” , IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 12, NO. 2, JUNE 2012
[ 7 ] Allyson L. Hartzell, et Al. “MEMS Reliability” , Springer, New York, 2011
[ 8 ] hypertext transfer protocol: //www.fandroides.com/wp-content/uploads/2014/04/8930-62341-How-does-smart-phone-accelerometer-work.png
[ 9 ] hypertext transfer protocol: //en.wikipedia.org/wiki/Failure_rate
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